Sex-linked genes are not silenced in fetal bovine testes expressing X-inactive specific transcript (XIST)

X-inactive specific transcript (XIST), which is thought to be the central f actor for the X-inactivation process in female mammals, is known to be expr essed in males during spermatogenesis. Our studies have shown that XIST is not only expressed in adult bovine testis but is also expressed in fetal, n ewborn, and prepubertal testes long before spermatogenesis is established. To determine whether the XIST expressed in fetal testes is involved in sile ncing the genes on the X chromosome, we investigated the status of X-linked genes, including glucose-6-phosphate-dehydrogenase (G6PD), hypoxanthine ph osphoribosyl transferase (HPRT), and X-linked zinc finger protein gene (ZFX ), in fetal bovine gonads at the developmental stage, when meiosis is initi ated in fetal ovaries in this species. Reverse transcription and a semiquan titative polymerase chain reaction based on the optical density of each gen e-specific band relative to that of the coamplified Quantum RNA 18S Interna l Standard (Ambion, Austin, TX) showed that the XIST gene was expressed in the testes of approximately 90-day-old fetuses and was silent in all their nongonadal organs tested, although at a significantly lower level than that in fetal organs of female fetuses. Our observation that the expression of X-linked genes in the fetal testis was comparable to that in male nongonada l organs, in which X inactivation does not occur, indicates that the low le vel of XIST, or XIST-like RNA, expressed in the fetal bovine testis is not involved in silencing X-linked genes. (C) 2001 Wiley-Liss, Inc.